1. Field of the Invention
This invention relates to switched reluctance drive systems. In particular, it relates to such systems operated with a limited duty cycle on a supply system with a limited capacity.
2. Description of Related Art
The characteristics and operation of switched reluctance systems are well known in the art and are described in, for example, xe2x80x9cThe Characteristics, Design and Application of Switched Reluctance Motors and Drivesxe2x80x9d by Stephenson and Blake, PCIM""93, Nxc3xcrnberg, Jun. 21-24, 1993, incorporated herein by reference. FIG. 1 shows a typical switched reluctance drive in schematic form, where the switched reluctance motor 12 drives a load 19. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. To this end, a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The rotor position detector 15 may take many forms, including that of a software algorithm, and its output may also be used to generate a speed feedback signal.
Many different power converter topologies are known, several of which are discussed in the Stephenson paper cited above. FIG. 2 shows one of the most common configurations for a single phase of a polyphase system in which the phase winding 16 of the machine is connected in series with two switching devices 21 and 22 across the busbars 26 and 27. Busbars 26 and 27 are collectively described as the xe2x80x9cDC linkxe2x80x9d of the converter. Energy recovery diodes 23 and 24 are connected to the winding to allow the winding current to flow back to the DC link when the switches 21 and 22 are opened. A capacitor 25, known as the xe2x80x9cDC link capacitorxe2x80x9d, is connected across the DC link to source or sink any alternating component of the DC link current (i.e. the so-called xe2x80x9cripple currentxe2x80x9d), which cannot be drawn from or returned to the supply. In practice, the capacitor 25 may comprise several capacitors connected in series and/or parallel and, where parallel connection is used, some of the elements may be distributed throughout the converter.
FIG. 3 shows typical waveforms for an operating cycle of the circuit shown in FIG. 2. FIG. 3(a) shows the voltage being applied for the duration of the conduction angle xcex8c when the switches 21 and 22 are closed. FIG. 3(b) shows the current in the phase winding 16 rising to a peak and then falling slightly. At the end of the conduction period, the switches are opened and the current transfers to the diodes, placing the inverted link voltage across the winding and hence forcing down the flux and the current to zero. At zero current, the diodes cease to conduct and the circuit is inactive until the start of a subsequent conduction period. The current on the DC link reverses when the switches are opened, as shown in FIG. 3(c), and the returned current represents energy being returned to the supply. This ability of a switched reluctance machine to allow energy to be returned to a supply circuit has advantages. For example, U.S. Pat. No. 5,705,918, incorporated herein by reference, discloses a generator that can transfer energy from a high-voltage bus to a low-voltage bus in order to increase generating efficiency.
The shape of the current waveform of a switched reluctance drive varies depending on the operating point of the machine and on the switching strategy adopted. As is well-known and described in, for example, the Stephenson paper cited above, low-speed operation generally involves the use of current chopping to contain the peak currents, and switching off the switches non-simultaneously gives an operating mode generally known as xe2x80x9cfreewheelingxe2x80x9d.
Switched reluctance drives are typically driven from the mains electricity supply. Some drives, however, do not have a fixed connection to the public electricity supply because they are installed on, for example, marine or automotive equipment. In these situations, the system is typically supplied by an alternator that is driven by a fossil-fuel-fired prime mover. A storage battery is usually provided to store sufficient energy to start the prime mover and to supply loads in excess of the generator capacity. It is re-charged by the alternator when there is sufficient generated capacity above that demanded by the system load.
With the alternator/storage battery systems described above, there is inevitably a compromise between capital cost, weight and performance. While the designer wishes to have a system capable of supplying any or all loads without the voltage on the system dropping, this can only be done by increasing the capacity of the battery and/or the alternator. This increases the capital cost of the system and the weight, which in turn leads to increased running cost and/or reduced dynamic performance from the boat or vehicle. A particular problem arises when a large load is intermittently operated, especially when the system is already supplying other loads that are sensitive to voltage fluctuations. For example, vehicle or cabin lighting using incandescent filaments is a load that is well-known to be sensitive to voltage fluctuations and indeed it is common for a slight dimming to occur when another load is switched onto the same supply bus. Where the load has a duty cycle of, say several seconds on followed by some tens of seconds off, this can be irritating to the eye.
There is therefore a need for a method of intermittently operating a drive on a limited capacity bus without causing significant voltage disturbance.
According to a first aspect of the invention, there is provided a switched reluctance drive comprising: a rotor, a stator having a winding, and a controller having means for selectively connecting either of a first and/or a second voltage source to supply the winding, and an energy return path between the winding and the second voltage source to allow returned energy to be transferred from the winding to the second voltage source when the first voltage source is used to supply the winding, thereby to charge the second voltage source.
An advantage of this drive is that energy is transferred from the winding to charge up a second voltage source for intermittent use.
The second voltage source may be greater than the first voltage source. Preferably, the second voltage source is charged up to a predetermined value, for example, two or three times that of the first voltage source.
The first and second voltage sources may be connected in series or in parallel. The first and second voltage sources may each include a capacitor connected across it in parallel.
The energy return path may comprise a diode that is connected between one end of the winding and the second voltage source in such a way as to transfer energy from the winding to the second voltage source.
The means for selectively connecting either of the first and/or the second voltage sources to supply the winding may comprise a pair of switches arranged in parallel, the first switch being connected between the winding and the first voltage source and the second switch being connected between the winding and the second voltage source, so that when the first switch is opened and the second switch is closed, the second voltage source can be used to supply the winding. A third switch may be provided for connecting the winding to a common terminal of both of the first and second voltage sources.
The means for selectively connecting either of the first and/or the second voltage sources to supply the winding may comprise a change-over switch that is operable in one position to connect the first voltage source to supply the winding and in another position to connect the second voltage source to supply the winding. The winding may be connected between and in series with a pair of switches.
According to another aspect of the invention, there is provided a method of operating a switched reluctance drive comprising a rotor and a stator having a winding, the method comprising: connecting a first voltage source to the drive so as to supply the winding; switching the first voltage source on and off across the winding; transferring energy from the winding when the first voltage source is switched off to a second voltage source, thereby to charge the second voltage source, and selectively connecting the second voltage source so as to supply the winding.
Preferably, the step of transferring is conducted until the second voltage source is charged up to a predetermined value, preferably higher than the voltage rating of the first voltage source, for example, two or three times that of the first voltage source.
The first and second voltage sources may be connected in series or in parallel. The first and second voltage sources may each include a capacitor connected across it in parallel.
The step of transferring may involve directing energy from the winding to the second voltage source via an energy return path that comprises a diode that is connected between one end of the winding and the second voltage source in such a way as to transfer energy from the winding to the second voltage source.
The winding may be connected in series with and between a pair of switches and the step of switching may involve switching the pair of switches between open and closed positions.
The method may further involve detecting when the second voltage source is charged to a predetermined level. Preferably, the method involves modifying the step of switching to reduce the energy returned to the second voltage source when it is charged to the predetermined level.